1. Field of the Invention
The present invention generally relates to a method of and an apparatus for detecting atrial fibrillation.
2. Discussion of the Related Art
The heart is the major muscle that functions as the primary pump for blood flow throughout the body. The heart contains two upper chambers called atria and two lower chambers called ventricles. The right atrium receives oxygen-depleted blood while the left atrium receives blood enriched with oxygen from the lungs. When the atria are full, the outlet valves within the heart open and the atria squeeze blood into the ventricles. The right ventricle then pumps oxygen-depleted blood to the lungs while the left ventricle pumps oxygen-enriched blood to all parts of the body. In this fashion, the heart functions primarily as a double sided pump.
The heart's internal pacemaker, known as the sinus node, signals the start of each heart beat. This signal originates in the right atrium in the sinoatrial node and travels simultaneously to the left atrium and down to the interatrial septum to the atrioventricular node. This electrical impulse results in a “p” wave on the electrocardiogram. This cycle of electrical stimulation that occurs normally is referred to as normal sinus rhythm. The contraction of the ventricles is preceded by QRS waves on the electrocardiogram (ECG), which is the electrical activity that begins ventricular contraction. This electrical activity is also often referred to as the “R” wave. The contraction of the heart occurs after the R wave. The impulse caused by cardiac contractility is transmitted through the arteries and is detected as a pulse. This pulse beat usually occurs from about 200 msec to about 700 msec after the R wave.
Many rhythm abnormalities may cause an irregular heart rhythm. Atrial fibrillation is a rhythm abnormality in which the atria do not contract normally. Instead, there is a continuously varying pattern of electrical activation of the atria resulting in a rapid highly irregular pattern of impulses reaching the atrioventricular node. The atrioventricular node acts as a filter and allows a reduced number of these impulses to reach the ventricles which results in a highly irregular heartbeat pattern. Since there is no organized electrical activity in the atrium, atrial fibrillation does not produce a p wave on the ECG.
Atrial fibrillation is one of the most common arrhythmias requiring medical attention. Atrial fibrillation may be caused by a number of heart conditions, such as coronary artery disease, myocardial infarction, heart valve abnormalities, and high blood pressure. These conditions may stretch or scar the atria, thereby causing irregularities in the heart system. Atrial fibrillation may also accompany lung problems or thyroid gland disorders and is also associated with significant morbidity and possible mortality. All persons, young and old, female or male, including the visually and/or sight impaired, may experience atrial fibrillation.
The most serious complication of atrial fibrillation is formation of a blood clot in the left atrium which may result in a stroke. The standard therapy used to prevent strokes in patients with risk factors for a stroke and atrial fibrillation is an anticoagulant, or blood thinner. Many people who develop atrial fibrillation, however, are unaware of their abnormal rhythm.
Recommendations have been made for people at risk of developing atrial fibrillation, to check their pulse periodically. Checking the pulse manually by palpation is often difficult for some people, especially the elderly, to do reliably. Therefore, use of a device that periodically automatically assess the heart rhythm and alerts the patient to the presence of atrial fibrillation would be helpful in getting patients with atrial fibrillation to be treated earlier. This may help prevent strokes in patients who are unaware that they have atrial fibrillation.
There are devices available that can be used by patients to screen for atrial fibrillation. The electrocardiogram (ECG) is the gold standard for determining if a person has atrial fibrillation. However, checking the ECG is cumbersome because it requires the person to place at least two electrodes on different body locations, such as both arms, an arm and a leg or an arm and the chest, or two locations on the chest. Also ECG monitoring at home often requires a technician and then a physician to read the ECG. The cost of this approach is prohibitive for the general population at risk of atrial fibrillation.
There are devices that can read the ECG automatically. However, they are easily compromised by a noisy signal, which is very common with ECG's. A noisy ECG signal can result in what is described as artifacts on the ECG signal. These artifacts can appear to be multiple R waves in an irregular pattern. These artifactual R waves will not have p waves preceding them and will, thus, result in the ECG meeting the criteria for diagnosing atrial fibrillation even though the true rhythm may be regular.
The use of blood pressure monitors and smartphones which can determine the time interval between pulse beats have been described. The blood pressure monitors rely on plethysmographic signals to detect the pulse, while smartphones can use the light transmittance through the skin to detect the pulse. The blood pressure devices, in particular, are able to detect the pulse reliably with artifacts rarely affecting the pulse signal. These modalities rely on assessing the regularity of the pulse rhythm which is irregular in atrial fibrillation. However, other rhythm abnormalities, such as extra heart beats may cause an irregular heart rhythm. These extra beats often follow normal beats that have both p and R waves on the ECG. Differentiating the rhythms due to extra heartbeats from atrial fibrillation can be performed most accurately by using the ECG.
Combining both the ECG recording and the pulse recording can improve the accuracy of detecting the true pulse beats. As mentioned previously, noise in either the ECG or pulse rhythm recording can result in artifacts that look like extra beats. Heart beats that occur with an adequate time interval following the previous beat to generate a pulse will always generate an R wave on the ECG and a pulse beat. Therefore, it is possible to use the pulse rhythm recording to help determine if what looks like an R wave on the ECG is due to a very premature R wave or an artifact since that electrical activity will not have a pulse beat. By deleting that electrical activity from the ECG, it is possible to generate a modified ECG that will have less electrical noise and very premature beats. This new modified ECG recording can then be analyzed for regularity. If it is regular, then the rhythm is not atrial fibrillation. If it is irregular, then the R waves on the modified ECG can be identified, and an attempt can be made to detect the preceding p waves. If the p waves are present, then the rhythm is not atrial fibrillation. If the p waves are absent, then the rhythm is atrial fibrillation.
The ECG recording and the pulse rhythm recording can be most easily compared by shifting the time of the ECG by from about 200 msec to about 700 msec so that the R waves occur at a later time. When the ECG time is shifted enough to account for the delay in generating the pulse rhythm, then the ECG and pulse recordings should have R waves and pulse beats occurring simultaneously. That is the time shift that can be used to generate the new modified ECG recording.
What is needed is a device that can be worn on a daily basis and can periodically take automatic pulse readings when the person is not moving so as to accurately determine if the heart rhythm is irregular.
What is further needed is for that device to inform the person when the automatic pulse reading showed an irregular rhythm and a combined ECG and pulse rhythm recording needs to be taken.
What is further needed is for the combined pulse and ECG recording to be analyzed to determine if atrial fibrillation is present and to inform the person of that result.
Methods for determining if an ECG waveform is noisy has been described in U.S. Pat. No. 8,639,316. However, in this patent publication, the presence of noise is determined by analyzing the properties of the ECG signal such as the morphology, amplitude or frequency content of the signal. This can also be applied to other physiological signals such as blood pressure waveforms. However, there is no mention of using a combination of physiological signals to determine if the ECG signal is noisy. There is also no mention of generating a new ECG recording by using a physiological signal to modify the ECG signal and then using that modified ECG to determine if atrial fibrillation is present.
U.S. Pat. Appln. Publ. No. 20130060154 describes a watch-like device that is worn on the wrist and can detect pulse signals which can be used to determine if atrial fibrillation is present. However, it does not describe obtaining recordings periodically and automatically when the person is not moving.